Effect of spark plasma sintering temperature on grain growth of nanocrystalline iron

Abstract

Pure iron powders were mechanically milled to produce nano crystallite with the grain size of 13 nm which was measured by transmission electron microscope. The nano crystalline iron powders were consolidated by Spark Plasma Sintering (SPS) in a temperature range from 300 to 900°C at 71 or 141 MPa. Optical (OM) and transmission electron microscope (TEM) observations were performed to measure the grain size. X-ray diffraction (XRD) was also done to measure the grain size by both the Scherrer method and the Wilson method. The grain size measured by XRD on the sintered specimen is smaller than that measured by TEM and OM by a factor about 1/3 and 1/100, respectively. The grain size of the as-milled powders increases to 33-50 nm and to 2-5 μm after the sintering at temperatures of 300-500°C and 600-900°C, respectively. The 600°C sintering produces a patchwork structure which is composed of the grains in 64 nm diameter and those in 2μm. The characteristic structure seems to be a result of the abnormal grain growth. In order to make a dense compact with grain sizes less than 50 nm, sintering should be done at temperatures lower than 600°C with higher pressures.